A soft polyurethane foam is widely used for a cushion material for a vehicle, a furniture mattress, a bedding, general goods, and so on owing to high cushioning properties thereof. The soft polyurethane foam is generally manufactured by reacting an organic polyisocyanate and a compound of two or more kinds (water is generally contained) containing active hydrogen under existence of a catalyst, a foam adjusting agent, and the other additives. Kinds of polyol, kinds of polymer polyol obtained by radical polymerizing acrylonitrile and styrene in the polyol, kinds of primary and secondary polyamines, water, and so on are used as the active hydrogen containing compound.
In general, a method is taken in which starting materials are mixed with a high-pressure foaming apparatus, injected into a mold to form, and thereafter, foamed cells inside the foam are forcibly communicated by using a compressor, in manufacturing of, for example, a foam for a vehicle cushion seat.
In the manufacturing process as stated above, a reaction speed of the foam and a quick removal of mold are necessary because shortening of a molding time, enabling low-energy and so on are required. On the other hand, low-density is required to reduce weight corresponding to a fuel consumption restriction in the manufactured polyurethane foam, particularly in the foam for the vehicle, and therefore, it is desired to enhance hardness of the foam to advance the low-density.
Conventionally, it has been proposed to compound various additives to correspond to the requirements as stated above. A cross-linking agent being one of the additives has a function to improve stability of the foam by reacting with isocyanate and increasing a molecular weight. In general, it is known that an amine-based cross-linking agent is used in the manufacturing of the polyurethane foam. For example, a method to manufacture the polyurethane foam by using an alkanolamine as the cross-linking agent is proposed (refer to Reference 1). However, a foaming reactivity is improved but a viscosity increasing speed also increases according to the increasing of the reactivity, and therefore, there have been problems in which fluidity just after the injection into the mold is lowered, the reaction proceeds before the foam composition diffuses details of the mold, and a failure ratio increases.
Besides, a technology improving physical properties of the polyurethane foam by using a specific catalyst composition composed of a tertiary amine catalyst and 2-methyl-1,3-propanediol is proposed (refer to Reference 2). In the Reference 2, it is described that an open cell (cells in the foam are communicated) effect is enabled because the cells are communicated by a low-pressure at the compression time after the removal of mold. However, there has been a problem in which enough hardness as a high-rebound polyurethane foam cannot be obtained when the low-density is enabled, and so on in this method.